A gas diffusion electrode has a feature in that a gas as a reaction substance is supplied to a surface of an electrode to proceed oxidation or reduction of the gas on the electrode, and it has been developed so far mainly for the application use of fuel cells, etc., and it has been started for the study on the use of the gas diffusion electrode for industrial electrolysis in recent years. For example, a hydrophobic cathode for conducting oxygen reduction reaction is utilized for an electrolytic production apparatus of hydrogen peroxide. Further, in alkali production and winning processes, hydrogen oxidation reaction is conducted as a substitute for the oxygen generation at the anode (hydrogen anode) or oxygen reducing reaction is conducted as a substitute for hydrogen generation at the cathode (oxygen cathode) by using a gas diffusion electrode intending to decrease the electric power consumption. It has been reported that depolarization is possible by collecting metals such as zinc, or substituting the oxygen generation at the anode as the counter electrode in zinc plating with hydrogen anode.
However, since the composition of the solution and the gas is not simple or the operation conditions are severe in the industrial electrolysis described above compared with the case of fuel cells, it gives rise to a problem that a sufficient electrode life and a sufficient performance can not be obtained.
Sodium hydroxide (caustic soda) and chlorine which are important as industrial materials are mainly produced by the sodium chloride electrolytic method. The electrolytic process has been changed through a mercury method of using a mercury cathode and a diaphragm method of using an asbestos diaphragm and a soft iron cathode to an ion-exchange membrane method of using an ion-exchange membrane for a diaphragm and using an activated cathode with a low overvoltage as the cathode. Meanwhile, the unit power consumption required for the production of sodium hydroxide has been decreased down to 2000 kwh per one ton. However, since the production of sodium hydroxide steel requires a great amount of power consumption, a further reduction for the unit power consumption has been desired.
The anode and cathode reactions in the existent electrolytic method are as follows, respectively:2Cl−=Cl2+2e (1.36 V)2H2O+2e=2OH−+H2 (−0.83 V)in which the theoretical decomposition voltage is 2.19 V.
In a case of using an oxygen cathode instead of taking place hydrogen generation reaction at the cathode, it is expressed as follows:2H2O+O2+4e=4OH− (0.40 V)in which power consumption can be decreased by 1.23 V theoretically and by about 0.8 V also in a practical range of current density and saving of unit power consumption by 700 kwh per 1 ton of sodium hydroxide can be expected. Accordingly, while the practical use of sodium chloride electrolytic process by utilizing a gas diffusion electrode has been studied since 1980 years, it is indispensable for the development of an oxygen cathode at a high performance and having a sufficient stability in the electrolytic system in order to attain the process.
The domestic or foreign situation regarding the oxygen cathode in sodium chloride electrolysis is detailed in “Domestic and Foreign Situations Regarding Oxygen Cathode for Sodium Chloride Electrolysis” (Sodium and Chlorine, vol. 45, p. 85-108 (1994)).
FIG. 1 shows a schematic view of a sodium chloride electrolytic cell 1 using an oxygen gas diffusion cathode which is practiced most generally at present.
An oxygen gas diffusion cathode 5 is disposed on the side of the cathode of a cation-exchange film 2 through a cathode solution chamber 6, and oxygen as a raw material is supplied from a gas chamber 7 at the back of the cathode. The oxygen diffuses in the cathode 5 and reacts with water in a catalyst layer to form sodium hydroxide. Accordingly, the cathode 5 used in the electrolytic method need to be a so-called gas-liquid separation type gas diffusion electrode that allows only the oxygen to permeate therethrough sufficiently and inhibiting the sodium hydroxide from leaking to the gas chamber. Oxygen gas diffusion cathodes proposed at present for use in sodium chloride electrolysis to satisfy such a requirement mainly include those of gas diffusion electrodes supporting a catalyst such as silver, platinum, etc. on an electrode substrate formed by mixing a carbon powder and PTFE and molding them into a sheet.
However, this type of the electrode described above involves several significant subjects. That is,
(1) Carbon used as an electrode material is easily deteriorated under the co-existence of sodium hydroxide and oxygen at high temperature to remarkably lower the electrode performance.
(2) It is difficult to prevent leakage of a sodium hydroxide solution generated along with increase of the liquid pressure and degradation of the electrode to the side of the gas chamber so long as the existent electrode is used.
(3) It is difficult to manufacture electrodes of a size necessary for a practical level (1 m2 or more) uniformly.
(4) While the pressure in the cathode solution chamber opposed to the gas chamber through the gas diffusion electrode changes depending on the height, it is difficult to provide a corresponding pressure distribution of the oxygen gas supplied.
(5) There is a solution resistance loss of the cathode solution, and it requires a power for the stirring thereof.
(6) Upon practical application, a remarkable improvement is necessary for existent electrolytic equipments.
The problem (1) can be coped with by the formation of a protective layer with silver catalyst powder or silver plating. On the other hand, a novel electrolytic method for solving the problems (2) to (6) has been proposed (refer to FIG. 2). An electrolytic cell 8 has a feature in that an oxygen gas diffusion cathode 9 is disposed in close contact with an ion-exchange membrane 10 (zero-gap structure), oxygen and water as the raw materials are supplied from the back of the electrode, and sodium hydroxide as a product is collected at the back or below the electrode. Since this is structured with two chambers comprising a cathode chamber served both as a cathode gas chamber and a cathode solution chamber, and an anode chamber, it is referred to as a 2-chamber method.
In a case of using the electrolytic method, the problem (2) can be solved, and separation between the cathode chamber (caustic chamber) and also the gas chamber is not necessary. Further, since the electrode is in close contact with the ion-exchange membrane in the structure, existent facilities of the ion-exchange membrane method can be used as they are, which can solve the problems (5) and (6).
The performance required for the oxygen gas diffusion cathode suitable to this electrolytic process is greatly different from that in the existent type and it is necessary to ensure a sufficient gas permeability, a sufficient hydrophobic property for avoiding flooding due to the solution of sodium hydroxide and a hydrophilic property for easily allowing the solution of sodium hydroxide to permeate in the electrode at the same time.
On the other hand, for collecting the solution of sodium hydroxide leaked to the back of the electrode, it is no more necessary that the electrode has a function of separating the cathode solution chamber and the cathode gas chamber. Accordingly, it is not necessary that the electrode is in an integrated structure and the size can also be increased also relatively easily to dissolve the problem (3).
Naturally, since it undergoes no effect of the change of the liquid pressure along the direction of the height, the problem (4) can not occur.
As such an electrode, JP 8-283979 A proposes a 2-chamber type gas diffusion electrode using a foamed or meshed nickel body as a substrate. Also in a case of using the gas diffusion electrode, since the formed sodium hydroxide moves not only to the back but also moves gravitationally in the direction of the height, sodium hydroxide stagnates inside the electrode in a case where sodium hydroxide is formed excessively to result in a problem of inhibiting the gas supply.
For solving the problem, Japanese Patent No. 3553775 proposes a method of disposing a hydrophilic layer between the ion-exchange membrane and the electrode.
As described above, while the gas diffusion electrode has been improved so as to be suitable to the industrial electrolytic system, in a case of operation at a high current density, even such an improved electrolytic cell structure for the 2-chamber method does not tend to obtain a sufficient primary electrolytic performance. It is supposed that since supply of the oxygen gas as the raw material to the electrode catalyst constitutes a rate-determining factor to hinder the reducing reaction of oxygen at the electrode.